Ordering • TCP sequence numbers uniquely order packets – One writer (sender) sets the sequence number – Reader (receiver) orders by seq number
• But recall distributed storage: may be more than one writer – One soluOon: If single server sees all the writes, can locally assign order in the order received, not sent – Recall parOOoned storage: What about ordering writes handled by different servers?
3
Time and distributed systems • With mulOple events, what happens first?
A shoots B B dies
4
Time and distributed systems • With mulOple events, what happens first?
B shoots A A dies
5
Time and distributed systems • With mulOple events, what happens first?
A shoots B
B shoots A
A dies
B dies
6
Just use Ome stamps? • Need synchronized clocks • Clock synch via a Ome server
p
Time server S
7
CrisOan’s Algorithm • Uses a !me server to synchronize clocks • Time server keeps the reference Ome • Clients ask server for Ome and adjust their local clock, based on the response – But different network latency → clock skew?
• Correct for this? For links with symmetrical latency:
RTT = Tresp received – Treq sent Tnew local = Tserver + (RTT / 2) Errorclock = Tnew local – Told local
8
Is this sufficient? • Server latency due to load? – If can measure Tnew local = Tserver + (RTT + lag / 2)
• But what about asymmetric latency? – RTT / 2 not sufficient!
• What do we need to measure RTT? – Requires no clock dria!
• What about “almost” concurrent events? – Clocks have micro/milli‐second precision
9
Events and Histories • Processes execute sequences of events • Events can be of 3 types: – local, send, and receive
• The local history hp of process p is the sequence of events executed by process
10
Ordering events • ObservaOon 1: – Events in a local history are totally ordered Process / Host
time
• ObservaOon 2: – For every message m, send(m) precedes receive(m) Host 1
m Host 2
time
time
11
Happens‐Before (Lamport [1978]) • RelaOve Ome? Define Happens‐Before (→) : – On the same process: a → b, if !me(a)
